1. Field of the Invention
The invention relates to a method of pretreating sulphide ores or sulphide ore concentrates that contain bismuth in concentrations obstructive to further processing of the ores or concentrates, so as to enable the ores and concentrates to be processed for the recovery of their valuable metal contents, or at least facilitate such processing.
2. Description of the Related Art
Sulphide ores and sulphide ore concentrates exist which because of their high bismuth contents either cannot be used for the recovery of their valuable metal contents or must at least be processed with great care, for instance diluted with bismuth-lean materials. The problems are most noticeable with material that shall primarily be processed to produce refined copper, either pyrometallurgically or hydrometallurgically, since in both cases there is carried out an electrolytic treatment process in which bismuth has a highly disturbing effect and contributes towards the formation of sludge/slime with the risk of serious impurity inclusions. Furthermore, refined copper is required to include a maximum of 1 ppm bismuth. Bismuth content limitations are also found in respect of lead and tin products, which is 50-100 ppm in the case of lead and 100-500 ppm in the case of tin.
The possibilities of handling bismuth in a pyrometallurgical copper producing process, for instance, are limited, because bismuth is a relatively noble element which is found in an elemental form in nature. Bismuth can be primarily separated as gaseous sulphide during the first stages of the copper process, i.e. during the smelting and converting stages, provided that good contact is obtained between the copper matte and gas and that the smelt has a high temperature. Bismuth extraction is not favoured by high-grade matte and the degree to which supplied process air is enriched with oxygen. Despite the aforesaid possibilities of reducing the bismuth content, separate bismuth purifying stages are often carried out in a later process stage, for instance in vacuum-refining or soda treatment process stages, although this often results in poor bismuth yields. In the majority of copper smelters, the maximum permitted bismuth content of the copper anodes used in the electrolytic refining process is as low as 50 ppm, so as to enable the aforesaid problems concerning sludge formations and the serious impurity inclusions caused thereby, implying bismuth contamination, among other things, can be avoided to the best possible extent. This means that the intake of bismuth to the copper process must be limited, and therewith also the intake of bismuth containing material. It is even necessary to reject certain materials because although the extraction of bismuth during the pyrometallurgical treatment process is relatively constant, it is much too low. As will be readily understood, attention must be aimed primarily towards productivity and process economy, which implies both high-grade matte and oxygen enrichment, which as before mentioned are not the best conditions under which bismuth is eliminated during the copper process.
Pretreatment of copper concentrates has been proposed with the intention of reducing different troublesome impurity contents of the concentrate prior to its further processing. Partial roasting can be used to reduce the arsenic content, but the bismuth content is only influenced marginally. The arsenic content, antimony content and mercury content can be reduced by sodium sulphide leaching, although the bismuth content is not influenced to any great extent by this treatment. In CA-A-1057310, Outokumpu have proposed pretreatment in a rotary furnace at 750.degree. C. in a sulphur vapour atmosphere. This process eliminates 100% of the arsenic, 50-60% of the antimony, but only 20-30% of the bismuth content.
In EP-B-0138794, Boliden have proposed a method of eliminating primarily Sb from copper smelt material. The method includes a separate chlorinating stage with an essentially stoichiometric quantity of chlorinating agent in a rotary furnace at temperatures of 450-750.degree. C., wherein Bi is also removed to a substantial extent. Bio-oxidative pretreatment, bio-leaching, for selectively leaching bismuth from copper concentrates has also been proposed. The selectivity and utility of bio-leaching to this end, however, would seem to be completely dependent on the mineralogical composition of the concentrate, and hence its use is limited to the treatment of certain specific concentrates of favourable composition.